The present invention is concerned with mercury cadmium telluride semiconductor devices. In particular, the present invention is directed to an improved mercury cadmium telluride photodiode. For the purposes of this specification, the common chemical notations for mercury cadmium telluride, (Hg,Cd)Te or Hg.sub.1-x Cd.sub.x Te, will be used.
Mercury cadmium telluride is an intrinsic photodetector material which consists of a mixture of cadmium telluride, a wide-gap semiconductor (E.sub.g = 1.6 eV), with mercury telluride, which is a semimetal having a negative energy gap of about -0.3 eV. The energy gap of the alloy varies approximately linearly with x, the mole fraction of cadmium telluride in the alloy. By properly selecting "x," it is possible to obtain mercury cadmium telluride detector material having a peak response over a wide range of infrared wavelengths. High performance mercury cadmium telluride detectors have been achieved for wavelengths from about 1 to 30 microns.
Initial work in (Hg,Cd)Te began in the late 1950's. The activity was primarily concentrated in development of photoconductive (Hg,Cd)Te detectors. Increasing interest and use has developed, however, for (Hg,Cd)Te photodiodes.
The formation of pn junctions in (Hg,Cd)Te to produce a (Hg,Cd)Te photodiode is complicated by the small dissociation energy of mercury telluride in the alloy. The formation of pn junctions must not cause excessive dissociation of the mercury telluride, since this will adversely affect the electrical and optical properties of the resulting device.
Several techniques have been developed for forming n-type layers on a p-type body of (Hg,Cd)Te. One of the simplest methods involves the annealing of the p-type (Hg,Cd)Te body in the presence of mercury vapor for a few hours at 300.degree. C. This technique is described in U.S. Pat. Nos. 3,468,363 by Parker, et al.; 3,514,347 by Rodot, et al.; and in an article by Figurovskii, et al., Soviet Physics-Semiconductors, 3, 1572 (1969).
Still other techniques for forming n-type layers on a p-type body of mercury cadmium telluride are possible. Among the techniques are bombardment with protons, electrons, or mercury ions. These techniques create an n-type layer by creating a damage induced donor state. These techniques are described in Foyt, et al., "Type Conversion and n-p Junction Formation in Hg.sub.1-x Cd.sub.x Te Produced by Photon Bombardment," Appl. Phys. Lett., 18, 321 (1971); Melngailis, et al., "Electronic Radiation Damage and Annealing of Hg.sub.1-x Cd.sub.x Te at Low Temperatures," J. Appl. Phys., 44, 2647 (1973); and Fiorito, et al, "Hg-Implanted Hg.sub.1-x Cd.sub.x Te Infrared Photovoltaic Detectors in the 8- to 14-.mu.m Range", Appl. Phys. Lett., 23, 448 (1973).
Another technique for forming pn junctions in p-type (Hg,Cd)Te is described by Marine, et al., "Infrared Photovoltaic Detectors from Ion Implanted Cd.sub.x Hg.sub.1-x Te," Appl. Phys. Lett., 23, 450 (1973). This method involves aluminum ion implantation and a subsequent anneal at 300.degree. C. for one hour to form an n-type region in a p-type (Hg,Cd)Te body.
Still another technique of forming pn junctions in p-type (Hg,Cd)Te is described in U.S. Pat. No. 3,858,306 by Kloek, et al. In this patent, pn junctions are formed in a p-type body of (Hg,Cd)Te by depositing hot indium on the p-type body. The hot indium forms an alloy junction.
Many other references describe the performance or structure of (Hg,Cd)Te photodiodes in which an n-type region is formed in a p-type body. Among these references are T. Kohler and P. J. McNally, Optical Engineering, 13, 312 (1974); Spears, et al., Appl. Phys. Lett., 23, 445 (1973); and U.S. Pat. Nos. 3,845,494 by Ameurlaine, et al.; 3,904,879 by Amingual, et al.; and 3,930,161 by Ameurlaine, et al.
For some applications, a p-on-n structure is preferable for a (Hg,Cd)Te photodiode. Techniques for forming p-type layers on n-type (Hg,Cd)Te, however, are not as well developed as the techniques for forming n-type layers on p-type (Hg,Cd)Te. One common method of forming p-type regions in n-type (Hg,Cd)Te is by depositing a gold layer on a surface of the n-type body and then heating the body to diffuse the gold, thereby forming a region of p-type conductivity. This method is described in U.S. Pat. No. 3,743,553 by M. W. Scott, et al. Another technique involves the ion implantation of gold into n-type (Hg,Cd)Te and a subsequent low temperature-short duration heat treatment. This technique is described in my co-pending application Ser. No. 662,293 filed Mar. 1, 1976, which is assigned to the same assignee as the present application.
Further improvements in p-on-n (Hg,Cd)Te photodiodes are very desirable, particularly for photodiodes operating in the important 8 to 14 micron wavelength region. Previous (Hg,Cd)Te photodiodes operating in this wavelength region have had a low zero bias resistance. This resistance has been lower than that necessary for optimum detector performance.